GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

R-38 low slope, vented roof Charlotte, NC.

user-7688267 | Posted in General Questions on

I’ve read the article about this here, but it is more about blown in or batt insulation.

I am adding a 16×32′ addition to the back of my house and to keep the costs down I want to have a monoslope , low slope roof which is about 10′ at the high end and goes down to about 9′ at the low end. by the time I add 10″ of ceiling insulation and 4″ of floor insulation I will be 7′ 10″ height at the low end.

I can either use traditional 2×10 rafters: TPO membrane roof, OSB, then 1.5″ air gap then 8″ of 2″ Poly iso boards which will have thermal bridging 

OR i can use steel stud rafters which are only 6″ tall. however I am having trouble visualizing how I would layer the insulation and still have an air gap with this approach. seems like I would have to screw a 2×4 on top of the 2×6 metal rafter so I could attach the OSB to the rafters. therefore the 2×4 would thermal bridge to the 2×6 and thermal bridging woudl still be an issue?

is there a better way to go about this? is there a water proof alternative to osb so I wont’ have to worry about mold/condensation? how about painting the osb with copper coat green anti microbial before installing?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    If all of the insulation is above the roof deck there is no need to vent the deck- there is no condensation risk.

    To meet code on a U-factor basis in Zone 3 A only requires ducking under U0.030 a "whole assembly-R" of R33, that factors in the thermal bridging, but also the R-values of all layers, including the OSB, ceiling gypsum, interior & exterior air films, etc. See TABLE N1102.1. 4:

    It's also the case in zone 3 that with a minimum of R5 on the exterior the structural roof deck the wintertime moisture load of the roof deck is well constrained even at a total presumptive center-cavity R of R38. See TABLE R806.5:

    Layering a continous inch of polyiso between the OSB and TPO would give you R6, and with R30 rock wool between 2x8 rafters would bring the center-cavity R to R36, which has better condensation control margin that the minimum, and would have no problems meeting code minimum performance on a U-factor basis due to the R6 thermal break over the rafters. If the inspector doesn't do math or requires seeing a stamped sheet from PE showing the U-factor math, using 2" of EPS or 1.5" of polyiso above the OSB would meet/beat the R38 code min with R30 rock wool between the rafters, and would have even better dew point margin the roof deck.

    With sufficent R above the roof deck no air gaps of any type are required. Using a vapor permeable underlayment above the OSB for the foam would ensure that the foam could still dry toward the interior. Going with #15 roofing felt would be fine in most cases, but a fully adhered vapor permeable system (eg VaproShield) would also be fine.

  2. user-7688267 | | #2

    Dana thanks so much for the fast detailed answer. Well if I do not have an air gap then the new low slope roof will block all the exsisting soffit venting and I will have to add about a dozen static vents $15 each x 12 = $180 more cost plus labor.

    in my head I think any venting will carry away the hot surface temperature and mitigate thermal transfer, but maybe with the white TPO roof material that is good enough to keep an unvented roof cool?

    I thought polyiso was closed cell water proof foam. does it really need a vapor barrier to expel moisure?

    this is a photo of the back of the exsisting house (ignore the cabinets and toilets, doing a remodel) the covered deck in the far background is 10x10 and the concrete patio slabe is 16x22' so basically the exsisting low slope roof over the deck will be extended another 22' long.

  3. user-7688267 | | #3

    this article has great illustrations and helped me out a lot for anyone that searches for the same question.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |